Method of forming recordable optical element using low absorption materials

ABSTRACT

A method of forming a recordable element including a substrate and having on its surface, in order, an optical recording layer and a light reflecting layer, the optical recording layer having at least two sublayers of different compositions is disclosed. The method includes forming in a sputtering chamber on the substrate surface a first sublayer of a predetermined thickness by sputtering at least two elements having Ge and Te, or alloys thereof, in a flowing environment of a hydrocarbon gas and an inert gas wherein the flow rate of the hydrocarbon gas is selected relative to the flow rate of the inert gas to provide the first sublayer with an elemental R min  reflectivity in the range of 40-60% and forming in the sputtering chamber on the first sublayer a second sublayer of a predetermined thickness by sputtering at least two elements having Ge and Te, or alloys thereof, in a flowing environment of hydrocarbon gas and the inert gas, with the flow rate of the hydrocarbon gas being selected to be greater than when forming the first sublayer so that the elemental R min  reflectivity of the second layer is in the range of about 70-85%. The method further includes forming a reflecting layer on the second sublayer and selecting the thicknesses of the first and second sublayers, and the reflecting layer such that the reflectivity of the recording element is about or greater than 70% for a laser wavelength of about 780 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to U.S. application Ser. No. 08/399,787 filed Mar. 7,1995, entitled "Recordable Optical Element Using Low AbsorptionMaterials" by Tyan et al and U.S. application Ser. No. 08/482,718 filedJun. 7, 1995, entitled "Recordable Optical Element Using Low AbsorptionMaterials" by Raychaudhuri et al.

1. Field of the Invention

The present invention relates to methods of forming optical recordingelements, particularly to those that are useful as recordable compactdiscs.

2. Background of the Invention

There are many types of optical information carrying elements that areknown. One of the popular forms of optical information containingelement is the compact disc or CD. Digital information is stored in theform of low reflectivity marks or pits on an otherwise reflectivebackground. Stringent specifications on CD formats have been publishedby Sony and Philips, and these formats are used as standards throughoutthe world. One of the most important format requirements is thebackground reflectivity which is specified to be greater than 70% atabout 780 nm. This high reflectivity value is unusual when compared withother optical recording discs. In the latter case, the reflectivitynormally is designed to be low in order to improve the absorption of thewriting laser energy to facilitate the information recording process.

In CDs, the optical information is most often in the form of read onlymemory or ROM. Optical information is usually not recorded in real timebut rather is produced by press molding. In a typical process, therecording substrates are first mass produced by press molding using astamper containing the digital information to be reproduced. The pressmolded substrate is then overcoated with a reflective layer and thenwith an optional protective layer. In those areas having thedeformations or pits, the reflectivity is lower than in those areas nothaving the deformations.

It is desirable to produce optical recording elements which are capableof being recorded in real time and producing a record that mimics theconventional CD on read out. In this manner, information can be recordedon the CD and the CD can be read back by conventional CD player.

It has been difficult to produce such optical recording elements becausethe recorded elements have to meet the strict specifications for CD. Inparticular, it has been difficult to produce recordable elements thatwill meet the >70% reflectivity requirement.

One method for forming a recordable element that mimics conventionalmold pressed CD elements is to provide a transparent, heat deformablesupport having thereon, in order, a layer that absorbs recordingradiation and a reflective layer. When radiated through the transparentsupport, the reflectivity varies with the thickness of the absorbinglayer as a result of the light interference effect and 70% reflectivitymay be realized at several thicknesses (FIG. 1). When an absorbing layerof very small thickness (much less than that corresponding to R_(min))is used, the reflectivity is high, but such structure is not useful forrecording purposes because of low thermal efficiency. The reflectivelayer is a very effective heat sink. Most of the writing energy absorbednext to the reflector in the optical recording layer is conducted awayby the reflector. It is generally observed that the smallest usefulthickness is that which produces reflectivity in the neighborhood of thefirst minimum in reflectance. To produce useful recording elements,therefore, requires materials which will produce >70% reflectance withthickness larger than this minimum useful thickness. Such materials arecharacterized by low optical absorption coefficients, contrary to thematerials used in conventional recording structures where high opticalabsorption is preferred. These low absorption materials when used inconventional media structure without a reflector generally do notperform well. Adequate sensitivity and contrast can only be achievedwhen incorporated in a complete optical interference structure using thereflector. Thus, generally speaking, materials that are appropriate forconventional recording structure are not appropriate for recordable CDstructure, and vice versa.

Materials of this type based on organic dyes are described in U.S. Pat.No. 4,940,618, European Patent Application 0,353,393, and CanadianPatent 2,005,520.

One of the undesirable features of elements based on such organic dyesis their wavelength sensitivity. The desirable optical properties canonly be obtained at wavelengths near the absorption edges of such dyes.As a result, the reflectivity and other properties of such elementsdepend strongly on wavelength. It is very difficult to meet all thestringent CD specifications throughout the entire range of wavelengthsthat the CDs are designed to function. It is nearly impossible tooperate such elements using shorter Wavelengths which are to be used infuture generation CDs to increase the recording density.

However, U.S. application Ser. No. 08/399,787 filed Mar. 7, 1995included some non-dye media which have satisfied the CD specifiedreflectivity. One of the shortcomings of such non-dye media is theirrelatively low thermal efficiency. The power required to write with fullcontrast is significantly greater than that required for the dye basedrecording elements.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method for makingimproved recording elements with significantly higher recordingsensitivity and operable at a wide wavelength range while complying withCD specifications.

This object is achieved by a method of forming a recordable elementincluding a substrate and having on its surface, in order, an opticalrecording layer and a light reflecting layer, the optical recordinglayer having at least two sublayers of different compositions,comprising the steps of:

a) forming in a sputtering chamber on the substrate surface a firstsublayer of a predetermined thickness by sputtering at least twoelements having Ge and Te, or alloys thereof, in a flowing environmentof a hydrocarbon gas and an inert Gas wherein the flow rate of thehydrocarbon Gas is selected relative to the flow rate of the inert Gasto provide the first sublayer with an elemental R_(min) reflectivity inthe range of 40-60%;

b) forming in the sputtering chamber on the first sublayer a secondsublayer of a predetermined thickness by sputtering at least twoelements having Ge and Te, or alloys thereof, in a flowing environmentof hydrocarbon gas and the inert gas, with the flow rate of thehydrocarbon gas being selected to be greater than when forming the firstsublayer so that the elemental R_(min) reflectivity of the second layeris in the range of about 70-85%;

c) forming a reflecting layer on the second sublayer; and

d) selecting the thicknesses of the first and second sublayers, and thereflecting layer such that the reflectivity of the recording element isabout or greater than 70% for a laser wavelength of about 780 nm.

ADVANTAGES

The method according to this invention provides:

The composition and the thickness of the optical recording layer and thereflecting layer are such that the recording sensitivity for the elementis superior to a single layer disc of identical R_(min) reflectivity;and

The composition and the thickness of the optical recording layer and thereflecting layer are such that the recording sensitivity for the elementis superior to a single layer disc of identical R_(max) reflectivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a typical plot of the reflectivity vs. the thickness of anoptical recording element resulting from light intereference effects;

FIG. 2 is a schematic representation, in cross-section, of an opticalrecording element which can be made in accordance with the presentinvention;

FIG. 3 is a schematic representation of apparatus which can be used topractice the present invention; and

FIG. 4 is a plot of optimum recording power vs. reflectance of anoptical recording element made in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning first to FIG. 2, there is shown an optical recording elementmade in accordance with the present invention.

A conventional substrate 10 which can be used in accordance with theinvention typically can be made of polycarbonate,polymethylmethacrylate, or another polymer or glass.

On the substrate 10 are formed first and second sublayers 12a and 12bwhich will be discussed in more detail later. On the sublayer 12b thereis provided a reflective layer 14.

It is an important feature of the present invention that the reflectinglayer 14 and the first and second sublayers 12a and 12b of the elementare selected such that the reflectivity of the recording element isgreater than 70% in order to meet the CD specifications.

Turning now to FIG. 3 wherein apparatus 20, which can be used inaccordance with the present invention, is shown. A belljar structure 22,which is conventionally used in sputtering operations, is shown. Withinthe belljar 22 are two DC magnetron cathodes 24a and 24b. Upon thesemagnetron cathodes 24a and 24b will be disposed metal targets havingelements for forming the first and second sublayers 12a and 12b,respectively. Each of the magnetron cathodes 24a and 24b is connected toits own variable DC power supply 26a and 26b, respectively. Thesubstrate 10, which can be in the form of a disc, is .disposed on asubstrate holder 28 which, as shown, can rotate. This type of structureis well understood to those skilled in the art and further descriptionof it is not necessary for understanding the present invention. Theapparatus used in the invention is connected to two sources of gases.The two gas sources 30a and 30b are hydrocarbon gas, preferably methane,and an inert gas, preferably argon, respectively. Connected to the gassource 30a is a mass flow controller 32a and connected to the inert gassource 30b is a mass flow controller 32b. By adjusting the mass flowcontrollers, the flow rate of the gases can be controlled. Further,there are shown a turbomolecular pump 34 and a mechanical pump 36, bothof which in combination are used to evacuate the belljar and to maintainthe required pressure as is well understood in the art.

In order to practice the present invention, a substrate 10 is placed onthe substrate holder 28 which is positioned over the first and secondmagnetron cathodes 24a and 24b. On the magnetron cathode 24a there isprovided a target having a metal element having Ge, and on the othercathode 24b there is provided another target having Te. Alternatively,an alloy target of both Ge and Te can be provided on a single magnetroncathode. In operation, the power delivered to the magnetron cathodes 24aand 24b by power supplies 26a and 26b, respectively, is adjusted and theflow rates of the hydrocarbon gas and the inert gas are selected so asto form each of the sublayers 12a and 12b.

In accordance with the invention, the thickness of the optical recordinglayer, which includes the first and second sublayers and the reflectivelayer, are selected such that the R_(min) relfectivity of the recordingelement is about or greater than 70% for a laser wavelength of about 780nm.

More particularly, the first sublayer 12a is selected to have apredetermined thickness by sputtering at least two elements having Geand Te, or alloys thereof, in an flowing environment of a hydrocarbongas and an inert gas wherein the flow rate of the hydrocarbon gas isselected relative to the flow rate of the inert gas to provide the firstsublayer with an elemental R_(min) reflectivity is in a range of 40-60%.

The second sublayer 12b is formed in a similar manner on the firstsublayer 12a and so the details need not be set forth. Frequently inpracticing the present invention, the second sublayer 12b will beselected to be of a predetermined thickness and is formed by sputteringat least two elemental targets having Ge and Te, or alloys thereof, in aflowing environment of hydrocarbon gas and the inert gas, with the flowrate of the hydrocarbon gas being selected to be greater than whenforming the first sublayer so that the elemental R_(min) reflectivity ofthe second layer is in a range of about 70-85%. After this test is run,the second sublayers 12b can be formed on the first sublayer 12a beforeforming the reflective layer 14.

The elemental reflectivity of the first sublayer 12a is measured in thefollowing manner. A reflective layer, such as gold, is formed on thefirst sublayer 12a and the reflectivity then is measured by illuminatingthe first sublayer through the substrate 10 using a spectrophotometer at780 nm. For purposes of this disclosure, the reflectivity as measured ofthe first sublayer will be called "elemental reflectivity" since areflective layer is actually added onto the first sublayer. Of course,the final device would not have a reflective layer on the firstsublayer, but this is necessary in order to characterize the firstsublayer. After this test is completed, the first sublayer can bereplicated on numerous substrates without using the reflective layer. Ina similar fashion, the elemental reflectivity of the second sublayer 12bis measured. In other words, a reflective layer is added onto the secondsublayer after it had been directly deposited on the substrate. Thiswill also be termed "elemental reflectivity."

In general, the first sublayer will be selected so as to have a lowerelemental R_(min) reflectivity than the second sublayer. However, thetwo sublayers, when combined, will produce a reflectivity for therecording element (when it has a reflective layer on second sublayer) ofabout or greater than 70%. With these criteria, most of the light willbe absorbed in the first sublayer. Moreover, the thermal properties ofthe sublayers 12a and 12b are different. It is believed that thesublayer 12b has a lower thermal conductivity than sublayer 12a. In thisway, the sublayers 12b acts as an insulator or heat barrier to preventthe recording energy to be dissipated through the highly conductivereflecting layer.

Protective layers may also be used but will not be discussed since theyare not necessary for the practice of this invention. The substrate 10is transparent and light which illuminates the optical recording layer12 passes through the substrate 10.

Recording is accomplished by absorbing energy focused on the sublayers12a and 12b with a write laser. The focused laser beam heats the opticalrecording element to temperatures substantially above the roomtemperature and induces changes in the media. The likely changes mayinclude agglomeration of the metallic components in the layer, or thedissociation of material to form gaseous species which, in turn, causesthe deformation of the media package in the form of bubbles, voids, orpits, etc. Some distortion of the substrate 10 material might also beinduced. In any event, the combination of some or all of these changesforms marks which can then be read back by the focused read laser beam.The record thus consists of marks of relatively low reflectivity on abackground of relatively high reflectivity in relation to the read laserlight.

The preferred embodiment of the optical recording element is that of awritable compact disc (CD-R). The write and read lasers are of the laserdiode type and Generally operate in the infrared region between 770 and830 nm.

For a more complete explanation of the optical recording and play backprocesses as well as the construction of compact discs, see OpticalRecording, Allan B. Marchant (1990).

The substrate 10

The substrate 10 can be made from optically transparent resins or Glassas discussed above with or without surface treatment. The preferredresins for the FIG. 2 embodiment are polycarbonate and polyacrylates.The substrate 10 may include a guide groove for laser tracking.

The Reflective Layer 14

The reflecting layer 14 can be any of the metals conventionally used foroptical recording materials. Useful metals can be vacuum evaporated orsputtered and include Gold, silver, aluminum, copper, and alloysthereof. Gold is the preferred material.

The preferred method of fabrication for the sublayers 12a and 12b is DCsputtering. The preferred target contains both the Te and Ge. Thetargets can be prepared by melt casting or powder metallurgy techniques.Alternatively, a co-sputtering method can be used where two or moresputtering targets are used, some contain the Te and some Ge. Theatmosphere contains a sputter gas such as Ar, and a hydrocarbon gas suchas methane.

Layers containing Ge, Sb, Te, C, and H have been fabricated for opticalapplications ((Okawa Japanese Kokai 171,289 (1990), U.S. Pat. No.4,985,349, and European Patent Application 0290009 (1988)). These layerswere designed, however, to be used for an optical recording layer 12without reflectors. For such applications, it is desirable to havelayers which are highly absorbing. For example, Okawa teaches the use oflayers made with Q<35%, where Q=CH₄ /(Ar+CH₄) is the fraction of CH₄ inthe sputter Gas. Okawa in European Patent Application 0290009 (1988)reported that for a layers fabricated with Q=50% and any of the metalsin a long list including Te, Ge, and Sb, the complex optical index isabout 3.7-0.59 i. That layer, if incorporated in a structure as in FIG.3, Gives only 5.6% reflectance at the first interference minimum and44.5% reflectance at the first maximum. These low reflectivity valuesare inadequate for CD applications. Layers made with lower Q values suchas those suggested by Okawa are said to be even more absorbing andobviously not suitable for CD applications. Furthermore FIG. 5 of U.S.Pat. No. 4,985,349 clearly indicated that it was not possible to producelayers with (C+H) content higher than 40 atomic % even when a sputteringatmosphere consisted entirely of CH₄ (i.e., Q=100%). One skilled in theart will conclude from these teachings, therefore, that it is notpossible to produce a layer based on Ge, Te, Sb, C, H with opticalconstants suitable for CD applications even when a reflector is applied.

For the fabrication of each of the sublayers 12a and 12b, a GeTe alloytarget was sputtered in an atmosphere comprising CH₄ and/or otherhydrocarbons. Two alloy targets can also be used when forming each ofthe sublayers. This is advantageous since the sublayers can be depositedmore rapidly and can have more uniformity in thickness; however, each ofthe sublayers could also have been formed by simultaneous sputteringfrom two targets, each with only one element. A reflecting layer 14 wassputter deposited in an inert gas ambient.

In order to achieve the required reflectivity of about or greater than70% for the recording element, the target power and the flow rate andpressure of the reactive gas have to be controlled during the depositionof the recording layer so that the recording element meets the CDspecified reflectivity (>70%).

Turning now to FIG. 4 where a plot is shown of the optimum recordingpower vs. reflectance of a bilayer (sublayers 12a and 12b) recordingelement formed in the process described above where the second sublayer12b is formed with different flow rates. More specifically, thehydrocarbon gas is increased when the second sublayer 12b is formedrelative to that used when forming the first sublayer 12a while keepingthe power and other parameters constant.

Reference should now be made to Table 1, which shows that five differentdiscs were made. Each of these discs was made with parameters shown inTable 1. It should be noted that the thicknesses of the second sublayerswere adjusted to achieve the various reflectivities for the recordingelements using two different flow rates, one for each of the sublayers12a and 12b. Table 1 depicts information wherein a recording element wasmade having two sublayers 12a and 12b. The sublayer 12a is, of course,adjacent to the substrate 10 and was deposited on the substrate using aset of deposition parameters. The sublayers 12b was deposited under adifferent set of conditions than that of sublayer 12a. The reflectivelayer was finally added onto sublayer 12b to complete the recordingelement. Turning our attention to the recording element 1, sublayer 12awas formed with a thickness of 100 Å. It was made by sputtering at 100 Wa GeTe alloy target. Methane gas was flowed into the belljar at a flowrate of 10 standard cubic centimeters (SCCM) and at the same time argongas of 10 SCCM was flowed into the belljar. This process was continueduntil sublayer 12a was 100 Å thick. Similary, sublayer 12b was producedas described below. Using the same target and other operatingconditions, except that the methane flow rate was increased to 20 SCCMuntil the sublayer 12b was formed having a thickness of 300 Å. A goldreflective layer then was coated onto the sublayer 12b. This recordingelement was tested in a dynamic tester and found to have optimumrecording power (ORP) of 16 mW at 2.4 m/s and reflectivity of 72.4% asplotted in FIG. 4 and shown in Table 1. In a similar fashion, four otherdiscs were made using the parameters listed in Table 1. It should benoted that by varying the thickness of sublayer 12b and keeping thethickness of sublayer 12a fixed at 100 Å, recording elements wereproduced which had different reflectivities. Using the plot of FIG. 4,an R_(min) of 69% and an ORP of 10.2 mW canbe realized using the processparameters listed in Table 1. Of course, it is desirable to adjust theparameters slightly to have the lowest possible ORP with R_(min) for therecording element being at least 70%.

                                      TABLE 1                                     __________________________________________________________________________    Sublayer 12a                                                                  and 12b                        Element                                                 GeTe                                                                              Sublayer 12a                                                                           Sublayer 12b                                                                           property                                       Recording                                                                          Ar  Target                                                                            CH.sub.4                                                                          Thickness                                                                          CH.sub.4                                                                          Thickness                                                                          Reflec-                                                                           ORP                                        element                                                                            SCCM                                                                              Watt                                                                              SCCM                                                                              A    SCCM                                                                              A    tivity %                                                                          mW                                         __________________________________________________________________________    1    10  100 10  100  20  300  72.4                                                                              16                                         2    10  100 10  100  20  400  70.2                                                                              12                                         3    10  100 10  100  20  500  69.4                                                                              10                                         4    10  100 10  100  20  600  69.9                                                                              10                                         5    10  100 10  100  20  700  73.4                                                                              11                                         __________________________________________________________________________

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

    ______________________________________                                        PARTS LIST                                                                    ______________________________________                                        10               substrate                                                    12a              first sublayer                                               12b              second sublayer                                              14               reflective layer                                             22               belljar                                                      24a              magnetron cathode                                            24b              magnetron cathode                                            26a              power supply                                                 26b              power supply                                                 28               substrate holder                                             30a              gas source                                                   30b              gas source                                                   32a              mass flow controller                                         32b              mass flow controller                                         34               turbomolecular pump                                          36               mechanical pump                                              ______________________________________                                    

We claim:
 1. A method of forming a recordable element including asubstrate and having on its surface, in order, an optical recordinglayer and a light reflecting layer, the optical recording layer havingat least two sublayers of different compositions, comprising the stepsof:a) forming in a sputtering chamber on the substrate surface a firstsublayer of a thickness by sputtering at least two elements having Geand Te, or alloys thereof, in a flowing environment of a hydrocarbon gasand an inert gas wherein the flow rate of the hydrocarbon gas isselected relative to the flow rate of the inert gas to provide the firstsublayer with an elemental R_(min) reflectivity in the range of 40-60%;b) forming in the sputtering chamber on the first sublayer a secondsublayer of a thickness by sputtering at least two elements having Geand Te, or alloys thereof, in a flowing environment of hydrocarbon gasand the inert gas, with the flow rate of the hydrocarbon gas beingselected to be greater than when forming the first sublayer so that theelemental R_(min) reflectivity of the second layer is in the range ofabout 70-85%; c) forming a reflecting layer on the second sublayer; andd) selecting the thicknesses of the first and second sublayers, and thereflecting layer such that the reflectivity of the recording element isabout or greater than 70% for a laser wavelength of about 780 nm.
 2. Themethod of claim 1 wherein the first and second sublayers are formed bysputtering from a single GeTe alloy target or by co-sputtering fromelemental Ge and Te targets with different flow rates of the hydrocarbongas but the same sputtering power.
 3. The method of claim 1 wherein thereflecting layer forming step includes the step of sputtering elementsselected from the group consisting of Au, Ag, Cu, Al, and alloysthereof.
 4. The method of claim 1 wherein the two sublayers of differentcompositions being selected such that each of them have differentthermal and optical properties.
 5. The method of claim 1 wherein thefirst sublayer adjacent to the substrate has a lower hydrocarbon contentthan the sublayer adjacent to the reflecting layer.
 6. The methodrecording element of claim 1 wherein the substrate is polycarbonate.